feat: track exceptions in :async activejob adapter

[halfbyte]

Fixes #361

I have some doubts about this but I wanted to give it a shot.

• I am not 100% sure if the way this is installed will work with all of the various loader configurations
• I am not sure if I have set up the call to .notify correctly and if my usage of .flush is correct.
• I am not 100% sure if this would survive a user installing additional callbacks (I will try to add a test for that)
• Due to the way ActiveJob testing works, I'm not actually testing the :async adapter (I don't think that that's a huge problem as I am not monkeypatching anything)

Please have a look.

[joshuap]

This seems reasonable to me, assuming it works in a live Rails app. 😁

I left a question about the test harness.

[halfbyte]

@joshuap I've removed the :sync and the flush block, as I think both of them are not strictly necessary after reading your explanations. I should've looked at these options a bit more when copying the code over :)

I've also simplified the test and tested it in a rails app from the console.

[joshuap]

This is looking much better! Just one last question.

[joshuap]

[halfbyte]

@subzero10 Can I merge this?

[joshuap]

I'm good with merging this 👍

[subzero10]

Sure go ahead!